Author Topic: calibration of barometric pressure  (Read 168 times)

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Offline gszlag

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calibration of barometric pressure
« on: November 11, 2021, 12:36:00 PM »
I was reading an older post about calibrating our Fine Offset weather station barometers.The author's calculations were close to being correct - but purely by chance as the station elevations were relatively low and therefore, the errors that were made were relatively low as well.

Here are some of the errors we need to avoid if we want to calibrate our barometers correctly:

1. Don't mix up QNH/Altimeter with MSLP - these are not the same thing. They use different formulas.

2. Use the right calculator.

If your country uses QNH in the METAR report DO NOT use any of the keisan online calculators unless you intentionally want to calculate MSLP (Mean Sea Level Pressure) or calculate QFF. Again, do not confuse the two; QNH is not the same as MSLP/QFF!

Outside of North America, METAR generally reports QNH not SLP. QNH is not the same as the SLP used in North American METAR. If you are located for example in Europe, don't use a SLP calculator. Use a QNH calculator to compare with a Europe based METAR.

3. Watch those default settings.

Unfortunately, the popular keisan online calculators set the station elevation temperature at a default 15C. This is incorrect unless you are assuming your year round station temperature is always the same at 15C. This is OK if you live at sea level but if you live say on Mt. Everest at 8900 meters and you leave the default temperature at 15C year round (not likely!) then you are also assuming the sea level temperature is well over 70C in the keisan calculator!

Look at the cartoon graphic of the mountaineer. For the popular keisan conversion to sea level pressure calculator, he will tell you which input variables can be changed. Calculate and use the standard ISA standard temperature for your altitude instead of the default 15C in the calculator or play a trick on the mounaineer and convert the keisan pressure and temperature calculator to an ISA calculator just by setting altitude =0 and leaving the other variables unchanged.

Example: Calculate ISA temperature and pressure for an altitude/elavation of 200 meters using 15C as the default mean sea level temperature. See screenshot below.
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[EDIT] grammar | added sentence re: convert keisan to ISA calculator | added screenshot
« Last Edit: November 14, 2021, 02:09:37 PM by gszlag »
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